So-called "superhard" materials include materials such as diamond, cubic boron nitride, and perhaps others having a Knoop hardness of at least 3000 kilograms per square millimeter. Diamond is at the present time the hardest material known.
It is known to grow diamond at temperatures and pressures which are low relative to the so-called "high temperature and pressure" conditions by which synthetic diamond material has been manufactured for some time for industrial use. Several approaches have been successfully used. In each case, it is necessary to provide a substantial amount of energy for dissociating hydrogen molecules to generate hydrogen ions which then assist in the process for growing diamond crystals on a substrate. In the "hot wire" method, an electrically heated resistance wire provides the energy. In the chemical vapor deposition (CVD) method, a microwave plasma provides the energy. In the "arc assisted plasma deposition" method, a direct current (d.c.) arc serves this purpose.
Diamond material may be grown by means of the above methods on selected substrate materials for various purposes. One application of particular interest is the provision of a diamond film at the cutting edge of a cutting tool, such as for machining metal. Diamond far surpasses any other known substance for this purpose, due to its extreme hardness, wear resistance, and high thermal conductivity, coupled with a low coefficient of friction. Typically, a diamond wafer is grown on a substrate and then removed from it as a free-standing structure. The wafer is then cut by a laser into small chips, which are then each bonded at the edge of a respective cutting tool insert for use in conjunction with a cutting tool holder.
One problem with present diamond film material for cutting tools is poor fracture toughness, that being a tendency for the material to suffer bulk fractures, usually leading to a complete failure of a costly tool.
In U.S. Pat. No. 4,822,466 issued to Schachner et al Apr. 18, 1989 it is observed that the adherence of a deposited diamond film to the substrate can be improved by first depositing a thin film of certain metals or carbides, nitrides, or oxides thereof. It is also said that "By interrupting the diamond deposition process with equal intervals and depositing layers of the above mentioned materials a thick coating can be built up, consisting mainly of diamond and having good adhesion to the substrate". The introduction of these foreign materials into the growth process, however, significantly adds to the complexity of the manufacture of diamond film.